[PCW] Hull design

[Mark]

Thanks Malcolm,

Certainly what you say makes sense.  I've often heard certain designs that use <= 10:1 LWL/Beam ratio discussed in terms of needing more load carrying capacity as justification.  Perhaps that's just peanut gallery commentary, but somewhere along the line someone made the call that shallow draught was more important than hull efficiency.  I'm sure there may be other factors, interior space, engine access, diminishing returns, etc.  

Since it's a linear calculation, a 20% improvement in slenderness costs 20% increase in draught for the same volume displaced.  

Malcolm, since I have you on the line, do have any data on the windward dynamic lift that can be generated by the CS hull form.  This would relative to putting a kite on an unmodified power cat and trying to get any upwind VMG.  Certainly lots of people have sailed up to 50 deg to the wind on sailing hulls with daggerboards or keels, just curious how a power cat hull would do.  Thanks.


--- On Tue, 3/11/08, Malcolm Tennant <malcolm at tennantdesign.co.nz> wrote:

> From: Malcolm Tennant <malcolm at tennantdesign.co.nz>
> Subject: Re: [PCW] Hull design
> To: mark424x at yahoo.com, "Power Catamaran List" <power-catamaran at lists.samurai.com>
> Date: Tuesday, March 11, 2008, 7:05 PM
> Marks comments on draught are interesting. Generally when
> you are designing a power cat hull there are a number of
> variables involved. If you have set the LOA then you try to
> calculate the full load displacement, you are looking for a
> particular hull speed/length ratio, a particular
> displacement/length ratio and probably a prismatic
> coefficient within a given range. Basically what this means
> is that once you set the LOA the draught is fixed if you
> optimise all the other variables.
> 
> Of course you can do it the other way by setting the
> draught, optimising all the other variables and then the
> LOA will be what it will be to give you that draught.
> 
> Once you set one of your variables your choice of values
> for the other variables becomes severely limited.
> 
> Of course full load displacement is the most important.
> Once you have that everything else tends to follow.
> 
> Regards,
> 
> Malcolm Tennant.
> 
> MALCOLM TENNANT MULTIHULL DESIGN LTD
> PO Box 60513, Titirangi.
> Waitakere 0642
> NEW ZEALAND
> Ph: +64 9 817 1988
> e-mail: malcolm at tennantdesign.co.nz
> www.tennantdesign.co.nz
> www.catdesigners.com


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